Experience with variable-frequency drives and motor bearing reliability

Motor bearing life historically has been difficult to predict. Many variables influence bearing life in motors, including operating speed, mechanical fit, loading, lubrication, vibration, and environmental conditions. A large synthetic fibers producer recently discovered that motor shaft voltage induced by variable-frequency drives (VFDs) can add another significant variable to the formula. Based upon this experience, this paper will present theory, test data, and actual results quantifying electrical discharge machining damage to bearings initiated by VFDs. The paper will present several mitigation techniques and quantify their relative effectiveness and attributes. Other factors and interactions of the above-mentioned variables also will be discussed in relation to the total goal of improving motor bearing life.